This invention relates to a method and to a system for constructing large continuous concrete slabs using closely spaced, cast-in crack inducers.
Large concrete slabs such as commercial, retail and industrial floors, and continuous pavements such as concrete roadways and paths will crack during the hydration period due to drying shrinkage of the concrete and other effects if they are not detailed to accommodate the shrinkage strains. In the absence of shrinkage control joints, cracks will typically occur in concrete slabs and pavements in the first three months after placing, and these cracks will normally meander through the concrete at random locations.
Uncontrolled, visible cracks in concrete slabs and pavements are generally perceived by those observing them as ugly at best and as failures at worst. Furthermore, the uncontrolled cracks are weak regions, which may fail under load, and uncontrolled cracks will widen and crumble under heavy traffic.
To remedy this problem in a conventional manner, shrinkage control joints of various types are introduced to provide a structural break in an attempt to accommodate and control the concrete shrinkage in predetermined locations. Although vastly superior to uncontrolled cracking, conventional control joints are expensive to install, and they are often the first point of failure in floor slabs and pavements.
The control joints are vulnerable to damage in traffic areas, usually due to impact, and they become unsightly when the slab edges break away and when sealants fail. They can also be a hazard for pedestrians and some random cracks often still occur despite the installation of a pattern of control joints.
There are a number of different control joints that are typically specified by engineers in the construction industry to accommodate shrinkage cracking of concrete slabs and pavements. One of the most popular control joints is a saw cut that is installed once the concrete has cured to the extent that it will support a worker. The depth of a suitable saw cut is typically twenty five percent of the total thickness of the slab and the spacing is typically three to six meters. Such a joint does not prevent cracking, but attempts to limit cracking to the saw cut locations and generally attempts to control cracking to straight lines. To achieve a relatively smooth finish and to seal the joint, saw cuts are usually filled with a suitable elastomeric material.
Unfortunately, this method is time consuming and involves a worker revisiting the slab after it has set to install the saw cut, and yet again to install the sealant. The additional time and material add to the cost of preparing the concrete slab.
Other traditional and commonplace shrinkage control joints include formed dowel joints, keyed joints and tooled joints.
U.S. Pat. No. 6,092,960 relates to a concrete joint restraint system, which secures dowel bars to a support structure. Use of dowel bars for transferring shear loads at joints in concrete pavement is known, and may provide a means to transfer forces across a joint. Using the invention of this patent, however, requires additional time and materials, and the use of joints.
U.S. Pat. No. 5,857,302 provides a means for controlling concrete slab cracking near walls or columns. The patent describes an outwardly extending vane perpendicular to the wall or column before pouring the concrete. The vane is orientated in line with a saw cut, which is made after the concrete has set. Although this invention directs cracking in a straight line near walls or columns, additional time and labor are still required in making the saw cuts.